A porous honeycomb structure constituted by cell partition walls (ribs) forming an assemblage of a plurality of cells adjacent to each other and a honeycomb outer wall surrounding and holding the outermost cells constituting the circumference of the assemblage of cells is in wide use as a filter (a diesel particulate filter, i.e. a DPF) for trapping and removing the particulate matter contained in a particle-containing fluid such as diesel engine exhaust gas or a catalyst carrier for carrying a catalyst component for purification of harmful substance in exhaust gas. Also, development works are being pushed forward for a DPF (a DPF for catalyst regeneration) which comprises an oxidation catalyst carried on a conventional DPF and which can oxidize and burn the particulate matter deposited on the catalyst to continuously regenerate the catalyst. Refractory silicon carbide (SiC) is in use as one of the constitutional materials.
As such a honeycomb structure, for example, there is disclosed a porous silicon carbide-based catalyst carrier of honeycomb structure which is obtained in such way that silicon carbide as a starting material having a desired specific surface area and containing impurities is formed into an article of intended shape, dried, then fired in a temperature of 1600 to 2200° C. (see, e.g. Patent Document 1).
In the case of the catalyst carrier disclosed in Patent Document 1, in sintering (necking) by the recrystallization reaction of the silicon carbide particle per se, the silicon carbide component vaporizes from the surface of the silicon carbide particles and condensates at the contact area (necks) between the silicon carbide particles, thereby the necks grow to result in a bonded state. However, the vaporization of silicon carbide requires a very high firing temperature, which has invited high costs, and a material of high thermal expansion coefficient must be fired at a high temperature, which has led to a drawback of a low firing yield.
Also, when it is attempted to produce a filter of high porosity, particularly high porosity of 50% or more by the above-mentioned sintering in the recrystallization reaction of the silicon carbide particle per se, the sintering mechanism does not function sufficiently, thus the growth of necks is hindered, which has led to a drawback of a low strength of the resultant filter.
As the conventional technique to solve these problems, there is disclosed a porous honeycomb structure containing fire resistant particles of aggregate, particularly silicon carbide and metallic silicon, and the production method (see, e.g. Patent Document 2). According to such production method, a porous honeycomb structure can be produced at relatively low firing temperatures and low costs, one with high thermal conductivity and high strength can be obtained. Also, in compounding, addition of a pore former enables a resultant porous honeycomb structure to be high porosity.
High porosity is desired for such honeycomb structure from the viewpoint of lowering pressure loss. As a method for making a honeycomb structure to be higher porosity, there is a method where organic pore formers such as starch and foaming resin are added in larger amount to a raw-material mixture for porous materials containing silicon carbide particles and the like, followed by burning off the pore formers in firing.
However, when porosity is to be increased, the amount of pore former to be added becomes large, in the case where a large amount of organic compound type pore former is added, combustion heat becomes large as well as the amount of gas such as volatile organic substance and carbon dioxide generated in a degreasing (calcinating) stage becomes large. There are instances that the calcinated (degreased) article or fired article obtained under such conditions has cracks, tears and cuts due to gas generation and combustion heat, and defects such as large pores caused by agglomeration of organic pore former added largely, and formation of faulty portions which do not exhibit filter functions and cause leakage of fluid. Also, in the case of using an organic pore former, although the porosity can be increased by an increase in the amount of pore former added, there has been a problem that pore diameter becomes large at the same time.
In order to solve the above-described problems, there is disclosed a method that inorganic micro balloons containing Si and Al and a compound containing alkaline earth metal are added to a raw-material mixture containing silicon carbide particles and metallic silicon, then formed into an intended shape, the resultant green body is calcinated and fired to melt the micro balloons to obtain a porous article of porous structure where an oxide phase containing Si, Al and alkaline earth metal is present on surfaces of and/or at circumferences of the silicon carbide particles and/or the metallic silicon (see Patent Document 3).
However, the method described in Patent Document 3 must add a large amount of alkaline earth metal to melt inorganic micro balloons and form communicating pores, resulting from adding a large amount of alkaline earth metal, there has been a problem that dimensional change becomes large in firing.
Patent Document 1: JP-A-6-182228
Patent Document 2: JP-A-2002-201082
Patent Document 3: WO 2003/082770 A1